1. Field of the Invention
The present invention relates to layers in semiconductor devices, and, in particular, to metal barrier diffusion layers.
2. Discussion of the Related Art
Integrated circuits fabricated on semiconductor substrates for very large scale integration typically require multiple levels of metal layers to electrically interconnect the discrete layers of semiconductor devices on the semiconductor chips. The different levels of metal layers are separated by various insulating or dielectric layers (also known as interlevel dielectric (ILD) layers), which have etched via holes to connect devices or active regions from one layer of metal to the next.
As semiconductor technology advances, circuit elements and dimensions on wafers or silicon substrates are becoming increasingly more dense. Consequently, the interconnections between various circuit elements and dielectric layers needs to be as small as possible. One way to reduce the size of interconnection lines and vias is to use copper (Cu) as the interconnect material instead of conventionally used materials such as aluminum (Al). Because copper has a conductivity approximately twice that of aluminum, the number of metal layers required will be less.
However, the use of copper as the interconnect material presents various problems. For example, there is currently no production-worthy etch process for Cu. This necessitates the use of a dual damascene xe2x80x9cinlaidxe2x80x9d approach. In the dual damascene approach, a dielectric or insulating diffusion barrier layer is deposited over a copper layer. The dielectric layer is then patterned, e.g., by conventional masking and etching techniques, to form a two-step connection having a narrower lower portion (or via portion) exposing desired connection areas on the underlying patterned metal layer and a wider upper portion (or trench portion) that will form the next layer of metal lines. The trench portion can be formed first or the via portion can be formed first (e.g., completely etching the dielectric layer down to the semiconductor substrate). Copper is then deposited to fill the two-step connection, such as by an xe2x80x9celectrofill (electrochemical deposition)xe2x80x9d process. The copper is then removed, e.g., by a chemical mechanical polish (CMP) process. The resulting structure is a via (the filled via portion) connecting the desired areas in the underlying metal layer with an overlying copper line (the filled trench portion).
In the dual damascene approach, the contact to the devices in the semiconductor substrate is usually made with Tungsten (W) plugs. Copper (Cu) can be used for subsequent metal layers. There could be variations in this regard with some manufacturers using Cu for some layers and Al for the others. However, when Cu is used, the copper atoms must be prevented from migrating or diffusing into adjacent dielectric oxide or other inter-layer dielectric layers, which can compromise their integrity as an insulator. Thus, when using Cu, a dielectric diffusion barrier (DDB) is typically formed between the top of the Cu metal line or layer and a subsequently deposited ILD layer to prevent copper atom migration into the ILD layer. The ILD layer is then etched to define a via and a next level of copper lines. The two-step connection for the via and line can be formed using, e.g., a xe2x80x9ctimedxe2x80x9d etch or an etchstop layer. A timed etch approach partially etches the ILD first. The etchstop approach utilizes an etchstop layer separating the ILD layer between the via and line. So the stack of the ILD would have a ILD layer (via level) followed by an etchstop layer at the desired height followed by an ILD layer (line level) on the etchstop layer. The via hole is defined first by photolithography. The vias are just vertical interconnections between a line above and a line below. So, when the lines are defined, we do not want the etch to continue below the desired vertical height. The etchstop layer precisely defines the line by not allowing etching beyond the etchstop layer.
Once the etching is done, Tantalum (Ta) or Tantalum Nitride (TaN) followed by a Cu seed layer is deposited, i.e. by PVD. This is followed by a fill of the line and the via hole with electrochemical deposition of Cu, such as using a process called electrofill by those skilled in the art. Excess copper is then removed by a Cu CMP step. Once the CMP step is completed, it is followed by CuOx reduction step and another DDB deposition step. Thus, DDB layers between dielectric layers and metal lines prevent copper atoms from diffusing into the ILD layers.
Typical materials used for the dielectric diffusion barrier are silicon nitride (SiN). Silicon carbide (SiC) is being considered since it can be made with a lower dielectric constant than SiN. For example, a SiC diffusion barrier can be formed using silane (SiH4) and methane (CH4) gases, with the flow rate of SiH4 varying between 100 and 500 sccm and the flow rate of CH4 varying between 9000 and 12000 sccm. The silicon carbide diffusion barrier is effective in preventing the migration of metal or copper atoms between adjacent metal layers. However, because silicon carbide is a semiconductor material, it can also exhibit a high leakage current, e.g., 300 xcexcA/cm2 at an electric field strength of 106 V/cm.
Accordingly, a barrier layer is desired that is effective in preventing the migration of metal atoms while also exhibiting low leakage current.
In accordance with the present invention, a barrier layer is formed with a silicon carbon (or carbo) nitride (SICN) material. A SICN barrier layer is effective both in preventing metal atom migration and in reducing leakage current, e.g., a leakage current of 90 nA/cm2 has been measured in a 106 V/cm electric field, compared to a 300 xcexcA/cm2 leakage current using a SiC barrier layer.
In one embodiment, the SICN layer is formed by adding ammonia (NH3) gas to silane (SiH4) and methane (CH4) gas in a plasma enhanced chemical vapor deposition (PECVD) chamber, with flow rates of the NH3 ranging from 500 to 4000 sccm. In other embodiments, the SICN layer is formed by adding inert gases to methylsilane (CH3SiH3), dimethylsilane (CH3(SiH2)2), trimethylsilane (CH3(SiH)3), or tetramethylsilane (Si(CH3)4) in conjunction with NH3. Deposition can be performed in either single station or multi-station PECVD chambers. The SiCN layer can also be formed using a high density plasma (HDP) deposition process with feed gases of 1) SiH4, C2H2, and N2 or 2) SiH4, CH4, and N2. Prior to forming the SiCN layer, the underlying metal or copper lines can be cleaned by using hydrogen or ammonia feed gases in PECVD or HDP chambers to remove the copper oxides. When cleaning in a PECVD chamber, the flow rates of NH3 or H2 range between 50 and 8000 sccm or higher, with the HFRF and LFRF power between 50 and 4000 W. When using an HDP chamber, typically flow rates of hydrogen range from 0 to 2000 sccm with a LFRF power range of 500 to 4000 W.
In other embodiments of the present invention, the SiCN layer can be used as an etchstop layer when forming a two-step connection using a dual damascene process. A first dielectric layer is deposited on the copper or metal lines, where the first dielectric layer thickness is approximately the height of the via. After the via is formed, the SiCN layer is deposited and a second dielectric layer is deposited over the SiCN layer. This second dielectric layer is then patterned and etched to form the copper line, thereby resulting in the two-step connection. The SiCN layer prevents the etching of the second dielectric layer to encroach on the first dielectric layer. The SiCN layer can also be used as a passivation layer to prevent scratches to the device.